At present, networks based on a 2nd generation global mobile communications technology or a 3rd generation global mobile communications technology (2G/3G, 2nd Generation/3rd Generation) are widely deployed in many areas, and may be regarded as full coverage to a certain extent.
Based on requirements of communications development, the Third Generation Partnership Project standardization organization launches new generation evolved network architecture, where a core network is a system architecture evolution (SAE, System Architecture Evolution) network, and an access network is an evolved universal mobile telecommunications system terrestrial radio access network (E-UTRAN) of a long term evolution (LTE, Long Term Evolution) network. A new generation evolved network has covered some metropolitan areas and traffic hotspots.
For ease of description, networks providing only PS domains, such as SAE/LTE, are referred to as pure PS domain networks. For different network bearer mechanisms, mobile communications services may be categorized into:
a service borne by a circuit switched (CS, Circuit Switching) domain, such as a voice telephone service, a short message service, and a supplementary service; and
a service borne by a packet switched (PS, Packet Switching) domain, including a data service and a session service, where the session service includes a voice over Internet protocol (VoIP, Voice over Internet Protocol) service.
In actual applications, a voice service may be borne by the CS domain, and may also be borne by the PS domain. Depending on different network coverage scenarios, in a communications process, a problem that a user needs to be handed over from a 2G/3G network to an LTE network may frequently occur. Therefore, how to maintain voice continuity in a network handover process is a technical problem that needs to be researched and solved.